I guess given you're right there all night, its quite possibly a prop. She's not sleeping a solid 12hr night if she's waking and shouting for a dummy regularly through the night, yk?
You say she's never slept longer than 40min for a nap? How was she after she woke from her 40min nap after 4hr A time?
I gather she's not an independent sleeper - going to sleep on her own?
Its actually normal to still have a DF and an NF at this age, so unless its an issue for you, I wouldn't worry too much about it.
Sorry to ask so many questions, just trying to wrap my head around it so I can think of what's happening - I think she must be OT on some level but she's been having good night, you say, up until recently so maybe not...